1. Field of the Invention
The present invention relates to methods for drilling boreholes in subterranean formations, particularly hydrocarbon bearing formations, and to drilling fluids for use in such drilling operations. More particularly, the present invention relates to oil or synthetic based drilling fluids, fluids comprising invert emulsions, and most particularly drilling fluid additives and methods for making such additives that facilitate or enhance emulsification, electrical stability or filtration properties of the drilling fluid.
2. Description of Relevant Art
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation.
Oil or synthetic-based muds, or invert emulsions, are normally used to drill swelling or sloughing shales, salt, gypsum, anhydrite or other evaporite formations, hydrogen sulfide-containing formations, and hot (greater than about 300 degrees Fahrenheit) holes, but may be used in other holes penetrating a subterranean formation as well. These non-aqueous based drilling fluids typically contain oil or a synthetic oil or other synthetic material or synthetic fluid (“synthetic”) as the continuous phase and may also contain water which is dispersed in the continuous phase by emulsification so that there is no distinct layer of water in the fluid. The term “oil mud” or “oil or synthetic-based mud” typically means an invert oil mud emulsion or invert emulsion. An all oil mud simply comprises 100% oil by volume as the liquid phase; that is, there is no aqueous internal phase. An invert emulsion drilling fluid may commonly comprise between about 50:50 to 95:5 by volume oil phase to water phase.
Most commonly, invert emulsions used in drilling typically comprise: a base oil or synthetic fluid for the external phase; a saline, aqueous solution for the internal phase (typically a solution comprising about 30% calcium chloride); and other agents or additives for suspension, fluid loss, density, oil-wetting, emulsification, filtration, and rheology control. With space at some well sites limited, such as on offshore platforms, and with increasing costs of transport of materials to a wellsite, there is industry-wide interest in, and on-going need for, more efficient and concentrated drilling fluid additives and for drilling fluids which can be formulated and maintained with minimal or fewer additives than common with prior art drilling fluids.
In response to this quest, U.S. Pat. No. 6,620,770, entitled “Additive of Oil-Based Drilling Fluids,” issued Sep. 16, 2003, to Jeff Kirsner, Jeff Miller, and Jon Bracken, taught a blend of a carboxylic acid terminated polyamide and a mixture produced by the Diels-Alder reaction of dienophiles with a mixture of fatty acids and resin acids preferably derived from the distillation of crude tall oil for providing emulsion stability and filtration control to invert emulsions and other oil or synthetic based drilling fluids for use in drilling boreholes in subterranean formations. That blend has a pour point at temperatures as low as about 20 degrees Fahrenheit with minimal solvent, enabling it to be transported in a highly (about 90% to about 100%) active state, which reduces the need to inventory products containing different solvents for compatibility with the drilling fluid. That blend also provides high levels of filtration control to a drilling fluid made with conventional emulsifiers at temperatures up to about 250° F. However, at higher temperatures (over about 250° F.), the blend requires other additives to stabilize fluid loss and after exposure to such high temperatures (over about 250° F.), the blend may have an undesirable thinning effect on the drilling fluid.
Consequently, the quest continues for more efficient and concentrated drilling fluid additives, and for drilling fluids which can be formulated and maintained with minimal or fewer additives than common with prior art drilling fluids, that can also tolerate or perform well at temperatures over about 250° F., and particularly at temperatures as high as about 350° F. or higher.